202 – II.3. BRASSICA CROPS (BRASSICA SPP.)
of HR genes and other markers have facilitated the detection of such genes in non-HR
B. napus plots and fields and multiple HR types in single HR crops. However,
measurement of the rate of outcrossing is complex as it can vary with the experimental
design, environmental conditions, cultivars grown, synchrony of flowering, insect
pollinator activity, local topography, and the relative size and arrangement of the donor
and recipient populations. Two types of designs have been used in these studies. In the
continuous design, the recipient population surrounds the donor, while in the
discontinuous designs the recipient populations are distributed in locations at increasing
distances from the pollen source (Hüsken and Dietz-Pfeilstetter, 2007). Using continuous
designs, over short isolation distances (0-30 m), researchers observed a rapid decline in
outcrossing rates as they sampled from the field edge into the recipient population
(Scheffler, Parkinson and Dale, 1993; Morris et al., 1994; Brown et al., 1996; Staniland et
al., 2000; Reboud, 2003; Dietz-Pfeilstetter and Zwerger, 2009, 2004). Examples from
such studies, conducted in Canada, the United Kingdom and the United States are given
in Table 3.6. These results underline the importance of determining outcrossing data
across the whole field and not just the level at a particular spot or distance into the field.
However, using commercially sized fields in a discontinuous design, Rieger et al. (2002)
found that fields situated within 100 m of the pollen source showed very little edge effect
while fields far from donor sources displayed a low and variable edge effect.
Table 3.6. Short distance pollen mediated gene flow from B. napus pollen donor
to recipient field/plots in Canada, the United Kingdom and the United States
Metres into recipient field Outcrossing % Reference, location and trial year
0.5 4.8
Scheffler, Parkinson and Dale (1993)
United Kingdom, 1991
1.0 1.5
3.0 0.4
6.0 0.11
12.0 0.016
24.0 0.004
36.0 0.001
0.0 2.0/3.5
Morris et al. (1994)
United States, 1992
east/west wind direction
0.3 1.0/1.5
0.6 0.75/1.2
3.0 0.65/0.6
4.6 0.50/0.6
0.0 0.70
Staniland et al. (2000)
Canada 1994-95
Data averaged over wind directions
and years
2.5 0.30
5.0 0.10
10.0 0.07
15.0 0.08
20.0 0.07
25.0 0.04
30.0 0.03
The mean rate of outcrossing at various isolation distances is a valuable statistic,
but the more important question might be “what is the maximum outcrossing that might
be expected at various distances?” There is now considerable evidence that the highest
rate of outcrossing that might be expected at 50-100 m is <0.5% and at 200 m the
maximum would be <0.1% (Tables 3.7 and 3.8 and Figure 3.36).
In most of the small plot trials, arranged in a continuous design, the area occupied by
the donor population is small in relation to the recipient populations, the ratio being about
1:4. This unequal availability of pollen tends to dilute the amount of donor pollen
accessible to both wind and bee vectors. As a result, outcrossing rates reported for small
